As guest OS, FreeBSD runs fine in hardware-assisted VMs (HVM) on both Xen and KVM, and there are a couple different ways to get FreeBSD to run in a paravirtualised VM (PV) on Xen. Official support for PV domU (Xen) has even been added to FreeBSD-CURRENT.

If one needs to run multiple OSes at once, on the same machine, they are better off going with a slimmed-down Linux install running KVM, Xen, VMware Server (or even VMWare ESXi), or VirtualBox, and run the different OSes in VMs, including the one they will be using the most.

Virtualisation work in FreeBSD is happening at the "run multiple worlds on single kernel" level (jails / vimage), and from the guest side of things.

Until someone comes out of the woodwork and starts working on VM host support for FreeBSD, this is the best we're going to get.